This invention relates generally to heat exchangers and more particularly to the structure of plate-type air-to-air heat exchangers. Numerous types of heat exchangers have been utilized to effect the transfer of heat from one stream of gases to another. As heat energy becomes increasingly expensive, a wide range of applications is being found for air-to-air heat exchangers. Many of these applications involve a process or application that contaminates the air, requiring that it be exhausted after being heated and that fresh air be supplied and heated. Examples of this situation include the traditional single pass of air through clothes dryers and forced air heated buildings, in which fresh, ambient air of relatively low humidity is heated and circulated through the dryer or building. As the air is circulated, it is humidified and acquires solid contaminates such as dust and lint. Conventionally, the air is then exhausted to the outside, with a resultant loss of its energy content.
It would be preferable to recover a portion of this energy by heat transfer to the incoming replacement air. Unfortunately, most prior attempts to do so have proven economically infeasible. It is possible to design a heat exchanger having a relatively high heat transfer efficiency, measured only in terms of heat recovered. However, the effectiveness of such heat transfer has proven marginal at best when measured additionally in terms of the value of the heat recovered in proportion to the cost of the apparatus to effect recovery.
A primary problem in the design of air-to-air heat exchangers is to transfer as much input heat as possible from the exhaust airflow path to the incoming airflow path while minimizing the energy required to pump the air through the system. A preferred form of heat exchanger for air-to-air heat transfer is the plate-type counterflow heat exchanger. Plate type parallel and cross-flow heat exchangers have also been used, such as disclosed in U.S. Pat. No. 2,959,400 to Simpelaar, but are less efficient than counterflow heat exchangers. The counterflow arrangement yields higher heat transfer efficiencies but heretofore has required complex and awkward header arrangements such as those of U.S. Pat. No. 2,019,351 to Lathrop, U.S. Pat. No. 2,937,856 to Thomson, and U.S. Pat. Nos. 3,581,649 and 4,184,538 to Rauenhorst.
Such arrangements create bends or folds in the airflow path which cause momentum changes in the airflows. Consequently, added pumping energy is required to move the airflows through the heat exchanger. Abrupt changes of direction also encourage accumulations of solid contaminants within the heat exchanger. Such accumulations lead to plugging thereby reducing the efficiency of the heat exchanger and further increasing its operating cost due to impedence of the airflows through the device. Additionally, such devices are extremely difficult to clean. As a result, a primary design goal for counterflow heat-type exchangers has been to attain a plate arrangement and header configuration which yields high heat transfer efficiencies and yet minimizes the required fan energy, to compensate for pressure drop across the heat exchanger. The need to minimize bending of airflows and changes of cross-sectional area have also been recognized. However, no prior design of parallel plate-type heat exchanger has succeeded in achieving these goals.
Also of importance is the mass producibility of the device, particularly the ease of manufacturing and assembling the plates and headers. U.S. Pat. No. 2,937,856 utilizes complicated stampings. U.S. Pat. No. 2,019,351 utilizes a complicated enclosure and seals. Physical size changes in most type of heat exchangers for each application require custom design and manufacturing of many sizes of the various components. Consequently, the design, manufacture and assembly of heat exchangers is too expensive for many applications in which the value of heat recovered is low in proportion to the cost of recovery.
Accordingly, a need remains for an air-to-air heat exchanger which is both efficient and cost effective, even for relatively low-value energy recovery applications, such as to hot air dryers and forced air heating systems in buildings.